A rare nucleoside, 3-(3-amino-3-carboxy-n-propyl) uridine can be assayed in specific species of tRNA as a result of its reactivity with the N-hydroxy-succinimide ester of phenoxyacetic acid (HSP). This nucleoside is present in E. coli tRNAPhe, tRNAIle, tRNAVal(2), tRNAArg, and tRNAMet. A similar, if not identical, nucleoside is present in rat liver tRNATyr, tRNAmThr, tRNAIle, tRNAAsn, and tRNACys. The relationship between tRNA modification and growth rate will be measured in bacterial cells by the characteristic change in mobility of the tRNA on BD-cellulose chromatography after the nucleoside has reacted with HSP. The occurrence of this nucleoside in tRNA extracted from mammaliam cells under different growth conditions and from tumors will be determined. The effect of drugs which alter methionine metabolism (an amino acid which is the precursor of the side chain of X), such as ethionine and 6-mercaptopurine, on the synthesis of X will be studied.